PCR amplification and Southern blots analyses of md alleles. (a) PCR amplification of exon 12 from genomic DNA from all md alleles produces an approximately 0.5 kb fragment, except for md^2J. (b) Southern blot of PstI digest of md allelic series (md, md^2J, md^4J, md^5J, md^6J) genomic DNA probed with part of exon 12 and its 5′ intronic region. The md^2J revealed a new fragment of 0.7 kb and a lack of fragment of 1.4 kb corresponding to exon 12, whereas md showed a loss of a 1.1 kb fragment and a novel 0.6 kb fragment corresponding to the intronic region 5′ of exon 12. md^4J, md^5J, and md^6J showed fragments of similar size to the C3H/HeJ^+/+ control. (c) Southern blot of HindIII digest of md allelic series probed with cDNA for exons 2–9 shows novel restriction site in md^5J.